Cathodic sputtering is widely used for the deposition of a thin film, or layer, of material onto desired substrates. The sputtering process employs gas ion bombardment of a target material having a face formed of a material that is to be deposited as a thin film, or layer, on the given substrate. Ion bombardment of the target material not only causes atoms or molecules of the target material to be sputtered, it imparts considerable thermal energy to the sputter target assembly. This heat is typically dissipated by use of a cooling fluid circulated beneath, through or around a thermally conductive, non-magnetic, backing plate material that is positioned in heat exchange relation with the target material.
The target material and backing plate material form a part of a cathode assembly which, together with an anode, is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target material surface, dislodge the target material. The dislodged target material traverses the evacuated enclosure and deposits, as a thin film, or layer, on the given substrate. The substrate is normally located proximate the anode within the evacuated chamber.
In addition to the use of an electrical field, increased sputtering rates have been achieved by concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target material surface thereby increasing the number of electron-gas atom collisions in the annular region to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target raceway. Magnetron sputtering imposes considerable thermal energy upon the sputter target assembly, especially within the concentrated annular region of the target raceway. The corresponding temperature variations within the target assembly induce forces which encourage separation of the target material from the backing plate material; this is especially true with regard to the peripheral zone of the target assembly.
In order to achieve good thermal and electrical contact between the target material and backing plate material, these members are commonly bonded to one another by means of soldering, brazing, diffusion bonding, clamping and by epoxy cement and the like. These bonding methods typically involve imposition of high temperatures. Sputter target assemblies bonded by these methods can bow or bend at high sputtering rates, especially when a large difference exists between the coefficients of thermal expansion for the target material and backing plate material. In many cases, in order to minimize bowing and bending at high sputtering conditions, target materials and backing plate materials having similar thermal expansion coefficients are utilized. In sputter target assemblies with internal cooling channels, bowing and bending induces leakage; the typical bonding methods, described above, tend to deform, or otherwise partially constrict, the cooling channels. Additionally, known bonding techniques, as discussed above, result in undesirable grain growth in the target material or the resulting bond cannot withstand the stresses imposed by high sputtering rates.
In some prior art target assemblies, projections formed in either the interfacial surfaces of the target material or backing plate material are received in corresponding recesses or grooves of the other member to improve thermal conductivity. For example in Hillman U.S. Pat. No. 4,885,075, annularly arranged members of high thermal conductivity protrude from either the target material or backing plate material into a corresponding recess formed in the other member. Upon heating of the target assembly during sputtering, the protruding members expand radially to make contact with the recess wall to thereby improve heat transfer between the target material and the backing plate material.
Elastic and plastic deformation of backing plate members in target assemblies is minimized in accordance with U.S. Pat. No. 5,269,899 (Fan), of common assignment herewith, by the provision of mating teeth like projections disposed along the target material/backing plate material interfacial surfaces, with the mating surface of the backing plate material having a concave surface. Here again, upon heating of the target assembly, the target material teeth expand radially and make snug contact with the mating teeth on the backing plate material to enhance thermal conductivity.
U.S. Pat. No. 5,230,459 (Mueller et al.) of common assignment herewith teaches the use of diffusion bonding methods wherein one of the target material/backing plate material mating surfaces is first prepared with grooves or the like. During the diffusion bonding process, jagged portions of the grooves penetrate into the metal from the opposed interfacial surface and disrupt the formation of oxide and other bond inhibiting layers that may otherwise form along the target material/backing plate material interface to thereby improve bonding efficacy. The method results in a strong bond with increased shear resistance such that the target assembly can withstand thermal stress that otherwise may tend to result in debonding or target assembly warpage during use.
As stated above, the induced separation forces tend to concentrate adjacent a peripheral zone of the target assembly. Prior art joining techniques employ a conventional weld around the periphery of the interface of the target material and backing plate material. For the reasons mentioned above, conventional welding techniques, even along only the periphery zone, are undesirable.
Therefore, there remains a need in the art of sputter target assemblies for a method of bonding the target material to the backing plate material which will withstand the stresses imposed by high sputtering rates, will allow for use of materials with dissimilar thermal expansion characteristics, and will not induce grain growth or cooling channel deformation. Specifically, improvement in the bond strength at the periphery is desirable.